Variable resistor with shorted section



Aug. 4, 1959 E. w. DREWITZ EIAL 2,898,567

VARIABLE RESISTOR WITH SHORTED SECTION 2 Sheets-Sheet 1 Filed Dec. 31,1957 INVENTORS EDA/IN w. DREW/7'2 wc ro /2 R STE/N ATTORNEY Aug. 4, 1959E. w. DREWITZ ETAL VARIABLE RESISTOR WI TH SHORTED SECTION Filed Dec.31, 1957 2 Sheets-Sheet 2 INVFFITORS EDWIN l. DREW/T2 VICTOR STE/M BYATTORNEY United States Patent VARIABLE RESISTOR WITH SHORTED SECTIONEdwin W. Drewitz and Victor Stein, Flushing, N.Y., as-

signors to De Jur-Amsco Corporation, Long Island City, N.Y., acorporation of New York Application December 31, 1957, Serial No.706,334

16 Claims. (Cl. 33892) The present invention relates to a variableresistor, and in particular to one in which a preselected portion of theresistance member is adapted to be short circuited.

One of the most conventional types of variable resistor comprises anelongated resistance member, usually formed of resistance Wire Woundabout a form, over a surface of which a movable contact slides.Appropriate electrical connections are made to the movable contact andto one or both ends of the resistance member, depending upon whether thedevice is to be used to vary resistance per se or to function as apotentiometer, and the particular position of the movable contact alongthe resistance member will determine the values of resistance or voltagemade effective in the external circuit to which the resistor iselectrically connected. Ordinarily, each time the movable contactchanges its position along the resistance member the value of resistanceor voltage will correspondingly change.

In many applications it is desired that the resistance or voltage shouldchange with movement of the contact over a given range of movement andthen should remain constant even though an additional movement isimparted to the contact. In other words, over a given portion orportions of the range of movement of the contact, no change in voltageor resistance is desired. The particular portions of the range ofcontact movement where constant resistance or voltage is desired willdiffer from installation to installation both as to length and as tolocation along the resistance member.

One way of providing for such portions of constant resistance is to windthe resistance Wire on the form with gaps in the windings correspondingto the areas Where no change is required, and then to provide built-inmeans for maintaining electrical connection with the contact when thecontact is positioned within the gaps. This requires winding specialresistance members for each individual installation or application. Italso presents very difficult production problems where accuracy isrequired. After a particular resistance member is wound it must first bemounted in its housing and then electrically tested for functionaccuracy. If errors are detected the resistor must be disassembled and acompletely new resistance member wound.

In accordance with the present invention means are provided forutilizing standard resistance members and for effecting short circuitingof predetermined areas thereof after they have been assembled in theresistor. The short circuiting can be accomplished in the assembledresistor, can be adjusted to achieve the desired degree of accuracy, theshort circuited areas can be changed at will, and the arrangement issuch that the normal functioning of the resistor is not impaired in anyway. Variable resistors made according to the present invention can beused as standard unshorted units and may be converted to shorted unitsin the field and without requiring any special equipment. This is ofparticular importance to those, such as the Armed Forces, who use alarge number of variable resistors in different applications. Suchcustomers can purchase standard units for all of their applications,which units can be used as such for conventional applications and can beconverted to special applications involving short-circuited areas as andwhen desired.

To these ends the resistance member is mounted in a casing and the shortcircuiting of preselected areas of the resistance member is accomplishedby means of a separate and readily insertable and removable thinflexible element of high conductivity, such as metal foil. The shortingelement is placed over that area of the contactengaged surface of theresistance member which is to be short circuited. Mechanical means areprovided for causing the shorting element to conform to thecontactengaged surface of the resistance member, the contact thereforesliding over the metal foil when it is in the shorted area. The foil iseasily assembled to the resistance member, and its position and lengthmay be adjusted until the desired degree of accuracy is obtained. Whenshort circuiting is no longer desired at a particular area, the metalfoil may be removed or repositioned, and the resistor can then functionin conventional manner or with short circuiting along a different areaof the resistance member.

In order to facilitate extreme accuracy in locating the area to beshort-circuited, adjustable taps of the type shown in Takats Patent2,619,570 of November 25, 1952, assigned to the assignee of thisapplication, may be employed, these taps being adjustably positioned onthe resistor casing so that they accurately engage the resistance memberat points at the beginning and the end of the area to be shortcircuited. A strip of metal foil of Width corresponding to the length ofthe shorted section is inserted between the taps, serving as locatingmeans for the foil. The taps and the foil are preferably externallyelectrically connected in order to ensure proper short circuiting of thedesignated areas.

In order to facilitate the mounting of the metal foil against theresistance member and to cause the metal foil to closely conform to theconfiguration of the contactengaged surface of the resistance member,one end of the foil is adapted to be received in a groove in the casingclosely adjacent to the resistance member and conforming to theconfiguration thereof, and releasable mechanical means such as aresiliently expandible split ring engages the other end of the foil soas to clamp it in position against the resistance member.

To the accomplishment of the above, and to such other objects as mayhereinafter appear, the present invention relates to the construction ofa variable resistor and the means for short circuiting preselected areasthereof as defined in the appended claims and as described in thisspecification, taken together with the accompanying drawings, in which:

Fig. 1 is a top plan view of a variable resistor embodying the presentinvention; but with the top cover removed therefrom;

Fig. 2 is a cross sectional view taken along the line 2-2 of Fig. 1;

Fig. 3 is a cross sectional view taken along the line 33 of Fig. 2; and

Fig. 4 is a cross sectional view taken along the line 4-4 of Fig. 1.

The resistor here illustrated comprises a casing generally designated 2within Which the arcuate resistance member 4 is mounted with itsradially inner surface exposed for engagement by movable contact 6, thelatter being mounted at the end of an arm 8 which is fast on shaft 10rotatably mounted in the casing 2 and connected to an actuating arm 12on the exterior of the casing 2. The casing comprises a bottom Wall 14and an upstanding side wall 16, a cover 18, shown fragmentarily in Fig.

2, being removably attachable to the upper end of the wall 16 so as toclose the open top of the casing. On the bottom wall 14 is a ring 20 ofinsulating material provided with an outer groove 22 within which thelower end of the resistance member 4 is received, that resistance membercomprising a form or card of insulating material around which resistancewire is coiled. One or more rings 24, 26 of insulating material aredisposed radially outside the resistance member 4 and between it and thecasing wall 16. The lower end of the resistance member 4 does notcompletely fill the groove 22, for reasons which will shortly becomeapparent.

Mounted on the bottom wall 14 radially inside the ring 20 is aninsulating ring 28 provided on its upper surface with a plurality ofrecesses 3%) equally spaced along its length. I

The central portion of the casing is provided with an upstanding hub 32within which ball bearings 34 are received, the shaft being journaled inthe bearings 34 and extending thereabove and above the hub 32, as at10', the arm 8 being secured to the shaft portion 10 for rotationtherewith.

The movable contact 6 is defined by a plurality of thin resilientlyflexible brush wires mounted on conductive bracket 36, the latter havinga pair of fingers 38 extendmg inwardly therefrom and engaging collectorring 40 mounted on an insulating bushing 42 carried by the hub 32. Theexterior of the casing 2 is provided with a terminal block 44 from whichterminals 46 extend, one of these terminals being electrically connectedto the collector ring 48 and thus to the movable contact 6, others ofthe terminals 46 being connected to the ends of the resistance windingson the resistance member 4.

As thus far described the device is capable of use as a conventionalvariable resistor or potentiometer.

In order to provide for short circuiting of a desired area of theresistance member 4 a short circuiting element 48 is employed which isseparable from and insertable into the resistor structure. The element48 is thin and flexible and has a highly conductive surface. It isadvantageously formed of metal foil, but it could also be constituted bya non-conductive sheet to which a coating of conductive material hasbeen applied. The width of the element 48 corresponds to the desiredlength of the short circuited area of the resistance member 4. Theheight of the shorting element 48, as here disclosed, is closely thesame as the height of the resistance member 4. The shorting element 48is positioned in the casing 2 so as to engage that surface of theresistance member 4 over which the movable contact 6 slides at the areathereof to be short circuited, so that the contact 6, when it reachesthe area in question, will engage and slide over the shorting element 48instead of directly engaging the resistance winding on the resistancemember 4. The flexible nature of the shorting element 48 permits it toconform accurately to the configuration of the contactengaged surface ofthe resistance member 4, and the preferably foil-like thinness of theelement'48 ensures that the movable contact 6, when it moves onto or ofifrom the shorting member 48, will do so smoothly and without bounce, sothat no discontinuity in electrical connection will result.

As here specifically disclosed the lower end of the element 48 isreceived within the groove 22 in the base ring just inside theresistance member 4. It was for this reason that, as was set forthabove, the resistance member 4- and the sheets 24 and 26 did notcompletely fill that groove 22. The reception of the lower end of theelement 48 in the groove 22 will help to ensure that the shortingelement 48 will closely conform to the configuration of the surface ofthe resistance member 4 to be shorted.

As here specifically disclosed a split fibre ring 50 of insulatingmaterial is inserted into the casing 2 via the open upper end thereof,that ring having a depending flange 52 which overlies the upper portionof the inner surface of the resistance member 4 and conforms to theconfiguration thereof, the upper end of the shorting element 48 beinginterposed between the ring flange 52 and the resistance member 4. Aresiliently expandible metallic ring 56 is received within a groove 58in the radially inner surface of the ring 50 and has its upbent ends 60received within recesses 62 adjacent the ends of the insulating ring 50,the element 56 therefore expanding the ring 50 into engagement with theupper portion of the resistance member 4 and thus firmly clamping theupper end of the shorting element 48 in position. The upper portion ofthe ring 50 is provided with a radially outwardly projecting part 64defining a shoulder 66 (see Fig. 2) which rests upon the upper edge ofthe resistance member 4 and thus limits the degree to which the ring 50may be inserted into the casing 2. Since the lower edge of the ring 50is positioned above the path of the movable contact 6, its presence doesnot interfere with the functioning of the resistor.

As is clear from the above description, the element 48 may be readilyinserted into the casing 2 at any desired position, and since it may beremoved or repositioned once the ring 50 has been removed, which may beaccomplished merely by contracting it and lifting it out of the open topof the casing 2, it is apparent that the resistor of the presentinvention can be converted from conventional operation to operation withone or more shorted areas in a very facile manner. Moreover, the preciseposition of the shorting member 48 may be adjusted in order to ensurethat it is properly located.

In order to facilitate the precise location of the shorted area, it ispreferred to use, for each shorted area, in addition to the shortingelement 48, a pair of taps generally designated 68, which may be of theform shown and claimed in Takats Patent 2,619,570, above referred to.These taps comprise a body portion 70 having an elongated aperture 72therein, the length of the aperture 72 being at least equal to thedistance between remote edges of adjacent recesses 30 in the ring 28.Hence no matter where the body portion 70 may be placed on top of thering 28 along its entire length, at least one recess 30 will becompletely exposed through the aperture 72, A screw or other fasteningelement 74 is adapted to pass through the aperture 72 and into anappropriate recess 30, engagement between the fastening element 74 andthe recess 30 clamping the body portion 70 in place on the ring 28. Thebody portion 70 is provided with a finger 76 extending therefrom towardand into engagement with the exposed surface of the re sistance member4. The body 70 is provided with another finger 78, the tip thereof beingprovided with an aperture 80 for the reception of a suitable conductive'wire for the making of electrical connection thereto. In practice apair of taps 68 will be employed for each area of the resistance member4 to be shorted, and they will be positioned and adjusted until thepoints at which they engage the resistance member 4 accuratelycorrespond to the beginning and end of the area to be shorted. Once thetaps 68 have been properly positioned, the linear distance between thepoints where they engage the resistance member 4 will have beendetermined and a shorting element 48 will be cut from a sheet of foil sothat its width corresponds to that distance, and the foil sheet 48 willthen be inserted between the taps 68, and more specifically between thepoints where those taps engage the resistance member 4, all as may bestbe seen in Figs. 3 and 4. Thus the accurately positioned taps 68 willserve as locating means for the foil sheet 48. In this regard it may benoted that the existence of some slight clearance between the side edgesof the foil sheet 48 and the points where the taps 68 engage theresistance member 4 is ordinarily entirely permissible, particularlywhere, as is here specifically disclosed, the taps 68 are electricallyconnected to one another as by the leads 82 and 84. It is also preferredthat the shorting element 43 be externally electrically connected to thetaps 68, and to this end, as may best be seen in Figs. 3 and 4, theleads 82 and 84 are themselves soldered or otherwise secured at 86 tothe exposed surface of the shorting element 48. If desired, another lead88 may connect the taps 66 and the shorting element 48 to an appropriateterminal 46 on the exterior of the casing 2.

From the above it will be clear that preselection of one or more shortedareas located over any desired portions of the resistance member 4 canbe made at any time, that the actual physical accomplishment of theshort circuiting can be carried out on a continually adjustable basisuntil the desired degree of accuracy and precision has been achievedwithout in any way disturbing or requiring modification of the basicresistor components, and that when short circuiting has beenaccomplished the movable contact 6 will cooperate therewith in aneffective and satisfactory manner. Consequently standard resistor unitscan be purchased and stocked for all applications, whether shorted areasare desired or not, and can readily be converted to have shorted areasat whatever points are dictated by the particular system with which theindividual resistor is to be used when and as such modifications arecalled for. The resultant saving in initial cost, the time required toproduce the short circuited modifications, and the maintenance ofadequate spare parts for all installations, are quite considerable.Moreover, the ready availability of spare parts for specialinstallations is a factor which alone is of controlling significance inmany situations.

While but a single embodiment of the present invention has been heredisclosed, it will be apparent that many variations may be made in thespecific structural details thereof without departing from the inventionas defined in the following claims.

We claims:

1. In a variable resistor comprising a casing containing an elongatedresistance member having an inner surface and a contact movable alongsaid surface; the improvement which comprises a thin element of highconductivity and of predetermined width engageable with and covering apredetermined portion of said resistance member surface and conformingthereto, said contact engaging and moving along said element, and a pairof electrically connected taps mounted on said casing and engaging saidresistance member at spaced points therealong, said element beingpositioned between said taps and having a width closely similar to thespacing between said points, said taps thereby constituting a locatingmeans for said element.

2. The variable resistor of claim 1, in which said element is externallyelectrically connected to said taps.

3. In a variable resistor comprising a casing containing an elongatedcurved resistance member having an exposed surface and a contact movablealong said surface, the improvement which comprises said resistancemember being mounted on said casing with a groove in said casingadjacent and conforming to the curvature of said surface, a thin elementof high conductivity and of predetermined width engageable with andcovering a predetermined portion of said resistance member surface andconforming thereto in curvature, an end of said element being receivedwithin said groove, said contact engaging and moving along said element.

4. In the variable resistor of claim 3, a pair of electrically connectedtaps mounted on said casing and engaging said resistance member atspaced points therealong, said element being positioned between saidtaps and having a width closely similar to the spacing between saidpoints, said taps thereby constituting a locating means for saidelement.

5. The variable resistor of claim 4, in which said eiement is externallyelectrically connected to said taps.

6. In a variable resistor comprising a casing containing an elongatedresistance member having an exposed surface and a contact movable alonga predetermined path on said surface, the improvement which comprises athin element of high conductivity and of predetermined width separablefrom said resistance member, engageable with and covering apredetermined portion of said resistance member surface, including saidpath, and conforming thereto, and a resilient member releasably engagingan upper portion of said variable resistor and conforming to saidresistance member, the upper end of said element outside said path beinginterposed and clamped between said resilient member and said upperportion of said resistor, said contact engaging and moving over saidelement along said path.

7. In the variable resistor of claim 6, a pair of electrically connectedtaps mounted on said casing and engaging said resistance member atspaced points therealong, said element being positioned between saidtaps and having a width closely similar to the spacing between saidpoints, said taps thereby constituting a locating members for saidelement.

8. The variable resistor of claim 7, in which said element is externallyelectrically connected to said taps.

9. In a variable resistor comprising a casing containing a base and anelongated curved resistance member having an inner surface and a contactmovable along a predetermined path on said surface, the improvementwhich comprises said resistance member being mounted on said base, saidbase having a groove adjacent and conforming to the curvature of saidsurface, a thin element of high conductivity and of predetermined widthengageable with and covering a predetermined portion of said resistancemember surface, including said path, and conforming thereto incurvature, the lower end of said element being received within saidgroove, and a resilient expansible member engaging an upper portion ofsaid variable resistor and conforming to the curvature of saidresistance member, the upper end of said element outside said path beinginterposed and clamped between said expansible member and said upperportion of said resistor, said contact engaging and moving over saidelement along said path.

10. In the variable resistor of claim 9, a pair of electricallyconnected taps mounted on said base and engaging said resistance memberat spaced points therealong, said element being positioned between saidtaps and having a width closely similar to the spacing between saidpoints, said taps thereby constituting a locating means for saidelement.

ll. The variable resistor of claim 10, in which said element isexternally electrically connected to said taps.

12. A variable resistor comprising a base, an elongated resistancemember in said resistor and having an elongated surface adapted to beengaged by a movable contact, a series of spaced mounting parts on saidbase and extending along but spaced from said resistance member surface,said base having a groove immediately adjacent said resistance membersurface and conforming thereto, said groove being located between saidresistance member and said mounting parts, a pair of electricallyconnected taps mounted on selected mounting parts and engaging saidresistance member at predetermined spaced points, a sheet of thinconductive material positioned against said resistance member surfacebetween said tap points and having a width substantially equal to thedistance between said points, the lower end of said sheet being receivedwithin said groove, and a contact movable along said resistance memberand over said sheet.

13. A variable resistor comprising a base, an elongated resistancemember in said resistor and having an and extending along but spacedfrom said resistance member surface, said base having a grooveimmediately adjacent said resistance member surface and conformingthereto, said groove being located between said resistance member andsaid mounting parts, a pair of electrically connected taps mounted onselected mounting parts and engaging said resistance member atpredetermined spaced points, a sheet of thin conductive materialpositioned against said resistance member surface between said tappoints and having a width substantially equal to the distance betweensaid points, the lower end of said sheet being received within saidgroove, means engaging the upper end of said sheet and causing saidsheet to conform to and snugly engage said resistance member surface,and a contact movable along said resistance member and over said sheet.

14. The variable resistor of claim 13, in which said sheet engagingmeans comprises a resiliently eXpansible split ring movable into saidresistor from the end thereof opposite said base and having an outershoulder engageable with a coacting part of said resistor to limit thedegree to which said ring may be moved into said resistor.

15. In a variable resistor comprising a casing con taining an elongatedresistance member having an exposed surface and a contact movable alonga predetermined path on said surface, the improvement which comprises athin element of high conductivity and of predetermined width engageablewith and covering a predetermined portion of said resistance membersurface, including said path and conforming thereto, and a resilientmember engaging an upper portion of said variable resistor andconforming to said resistance mem her, the upper end of said element,outside said path, being interposed and clamped between said resilientmember and said upper portion of said resistor, said contact engagingand moving over said element along said path, and a pair of electricallyconnected taps mounted on said casing and engaging said resistancemember at spaced points 'therealong, said element being positionedbetween said taps and having a width closely similar to the spacingbetween said points, said taps thereby constituting a locating means forsaid element.

16. The variable resistor of claim 15, in which said element isexternally electrically connected to said taps.

References Cited in the file of this patent UNITED STATES PATENTS1,948,354 Lodge Feb. 20, 1934 1,987,118 Lodge Jan. 8, 1935 2,081,572Bagno May 25, 1937 2,448,783 De Giers et a1. Sept. 7, 1948 2,551,989Wilson May 8, 1951 2,573,122 Weber Oct. 30, 1951 2.806.928 Woods et a1.Sept. 17, 1957

